Molybdenum disulfide (MoS2) is usually recovered from molybdenite ore obtained from a variety of mine sites, including “primary” and “secondary” mine sites. Broadly speaking, molybdenite ore consists of silicified granite compositions having deposits of soft, black, and hexagonal MoS2 crystalline structures widely dispersed therein (e.g., in an average concentration of only about 0.03-0.6% by weight of the entire ore body). One of the largest sources of MoS2-containing ore (e.g., molybdenite) is the Henderson molybdenum mine near Empire, Colo. (US), currently operated by the Climax Molybdenum Company, although other mine sites throughout the world are able to produce large amounts of this material as well. The Henderson mine site is characterized as a primary mine and is capable of producing large amounts of molybdenite ore.
Molybdenum disulfide also may be recovered from secondary or by-product molybdenite obtained from secondary mine sites. For example, secondary molybdenite may be obtained from copper mines, wherein the secondary molybdenite is typically combined with copper-containing materials. In such cases, molybdenum disulfide concentrate is obtained as a by-product from copper sulfide-molybdenum disulfide separation processes.
In a typical processing operation, the molybdenite ore is initially subjected to a physical grinding process in which the ore is reduced in size to a plurality of small particles (e.g., having a typical particle size of about 100 U.S. mesh or less). The ore particles are then further treated to remove the desired MoS2 therefrom. This treatment step may be accomplished using a variety of techniques, including froth flotation extraction procedures that employ various hydrocarbon compositions and wetting agents known in the art for this purpose. As a result, the desired MoS2 may be effectively separated from ore-based waste materials (i.e., “gangue”), such as silica, silicates, clays, and other unwanted materials. The desired MoS2 compositions will, by virtue of their minimal weight and density levels compared with the gangue, be readily isolated in the flotation froth.
The resulting molybdenum disulfide concentrate (i.e., from either primary or non-primary sources, as noted above) may be dried and sized (e.g., by grinding and subsequent classification steps) to produce a molybdenum disulfide powder product having the desired grade and particle size. Exemplary grades of molybdenum disulfide include “technical,” “technical fine,” and “superfine” grades, although other grades are known and commercially available. Technical grades of molybdenum disulfide typically comprise about 98% (by weight) molybdenum disulfide, with the balance comprising various amounts of iron, molybdenum trioxide, water, oil, and carbon, depending on a variety of factors.
The particle sizes of the molybdenum disulfide powder product may also vary within a specified grade. For example, technical grade molybdenum disulfide powders may have median particle sizes in the range of about 15-20 μm, with a small percentage of particles being as large as 200 μm or so. Technical fine grades may have median particle sizes in the range of about 4-6 μm, whereas superfine grades may comprise median particle sizes in the range of about 0.9 to about 1.6 μm.
Molybdenum disulfide powder products may also be characterized by an “acid number” and by an “oil number.” The acid number of molybdenum disulfide typically refers to the amount or quantity of a base, such as potassium hydroxide (KOH), required to neutralize the acid in a defined quantity of molybdenum disulfide. The acid number is typically expressed as an absolute value and is the number of milligrams (mg) of potassium hydroxide (KOH) required to neutralize the acid in a 1-gram sample of molybdenum disulfide.
The acid number of a molybdenum disulfide product may vary from producer-to-producer and also tends to vary with particle size within a given grade. Generally speaking, the acid number increases with decreasing particle size. Thus, a plain technical grade molybdenum disulfide will typically have the lowest acid number, with the technical fine and superfine grades having progressively higher acid numbers.
The oil number is a measure of the residual oils contained in the molybdenum disulfide powder product. The residual oils may originate from a variety of sources, including the various froth flotation steps used during ore beneficiation, as well as from oils added to the molybdenum disulfide during various grinding or milling processes required to reduce the molybdenum disulfide powder product to the desired particle size. The oil number is typically expressed as a weight percentage of oil contained in the molybdenum disulfide powder product.
Depending on the ultimate application or use of the molybdenum disulfide powder product, it may be desirable or advantageous to reduce the acid number and/or oil number as much as possible. While various processes are known and may be used to reduce the acid and/or oil numbers, such processes typically involve additional processing steps, thus additional time and processing expense, before the molybdenum disulfide product will have the desired acid and/or oil numbers. Consequently, methods continue to be sought that will provide for the effective control of the acid and/or oil numbers in the molybdenum disulfide product while minimizing the need for additional time or process steps to achieve the desired product specifications.